1. Technical Field
This invention relates to a developing roller and an imaging apparatus comprising the same, and more particularly to a developing roller having a low compression permanent strain and an excellent resistance to toner fusion in use under high temperature, high humidity environment for a long time and an imaging apparatus comprising such a developing roller and capable of forming a good image.
2. Related Art
In the imaging apparatus of an electrophotographic system such as a copier, a printer or the like, a pressurized developing process is known as a developing method wherein a toner is supplied to a photosensitive drum keeping a latent image to visualize the latent image through the toner attached to the latent image on the photosensitive drum. In such a pressurized developing process, the photosensitive drum is charged, for example, to a certain potential and then an electrostatic latent image is formed on the photosensitive drum through a photolithography machine and thereafter the developing roller carried with the toner is contacted with the photosensitive drum kept with the latent image to attach the toner to the latent image on the photosensitive drum.
In the above pressurized developing process, the developing roller should be rotated while surely maintaining the closed state to the photosensitive drum, so that it has a structure that a semiconductive elastic layer made of a semiconductive elastomer, which is obtained by dispersing carbon black or metal powder into an elastomer such as polyurethane, silicone rubber, acrylonitrile-butadiene rubber (NBR), ethylene-propylene-diene rubber (EPDM), epichlorohydrin rubber (ECO) or the like, or a foamed body thereof is formed on an outer periphery of a shaft made from a good electrical conductive material such as a metal or the like. Also, a surface coating layer may be further formed on the surface of the elastic layer for the purpose of controlling a charging characteristic or adhesion property to the toner, preventing contamination of the photosensitive drum due to the elastic layer and the like.
Recently, imaging apparatuses using a low melting point toner are increasing with the advancement of colorization, high speed performance, high-quality picture and energy-saving in the imaging apparatus. When the low melting point toner is used, such a toner is damaged by repeatedly subjecting to compression or friction between the constitutional members to cause aggregation, fusing or the like of the toner, whereby a poor imaging such as fogging or the like is easily caused. That is, the toner is deteriorated at the time of ending the service life of the toner cartridge to make the charging characteristic non-uniform, so that there is caused a problem that the poor imaging is caused or the service life becomes short. Therefore, a low hardness developing roller hardly giving the damage to the toner is demanded, and there are known a developing roller in which a substrate (elastic layer) is softened (JP-A-2003-15404) and a developing roller in which a coated film (surface coating layer) is softened by compounding an adipic acid ester compound as a plasticizer (JP-A-2005-128067).
On the other hand, when the low hardness developing roller is used in the imaging apparatus, there is a problem that traces due to the pressing of the photosensitive drum, blade, feed roller or the like is easily caused on the surface of the developing roller. For this end, it is demanded to provide developing rollers having not only a low hardness but also a low compression permanent strain in recent years. However, materials having a low hardness tends to increase the compression permanent strain, so that it is very difficult to balance both the hardness and the compression permanent strain at a sufficiently low level (see JP-A-H08-190263, JP-A-2001-75354 and JP-A-2004-67726). Also, when the coated film compounded with t he adipic acid ester compound as a plasticizer is used, the coated film becomes soft, but there is caused a problem that the compression permanent strain becomes large but also the fusing of the toner is caused in the use under a high temperature, high humidity environment over a long time to produce vertical strips in the image.